While learning Japanese we students have a secrete weapon called カタカナ。We use katakana words even though there are perfectly fitting Japanese words which we obviously don't know. Just because we understand English better than Japanese we cant use English words written in カタカナ as means to overcome the hurdle at the moment.
So my question is how much カタカナ is acceptable by Japanese people?
This depends on the topic and the type of what you are writing, so it's impossible to generalize. An article about programming or Disney characters will naturally contain a lot of katakana words, whereas you probably want to intentionally avoid katakana words when you are writing a samurai novel. You can see a statistical analysis based on a newspaper corpus here.
Still, it can be said that sentences with too many katakana loanwords can look absurd (see examples here), and you eventually have to master how people normally describe each concept in Japanese. Meanwhile, of course it's perfectly logical to use some English for difficult concepts if you think your listener understands it. If I, a native Japanese speaker, said オプティミスティック (katakana for "optimistic") in a casual conversation, I would definitely sound snobbish, but that may not be the case with you.
